1. Field of the Invention
This invention relates to a fluid flow rate measuring device and a water meter that are capable of electronically measuring a flow rate of a liquid or a gas.
2. Description of the Related Art
There has been known an electronic water meter using a magnetic sensor or an LC resonant circuit. The water meter using the magnetic sensor is disclosed in Japanese Patent Application Publication No. 2008-224320, for example. The water meter using the LC resonant circuit is used in Europe in recent years. The water meter using the LC resonant circuit is hereafter explained.
FIG. 14 is a schematic drawing showing a structure of the water meter using the LC resonant circuit. The water meter is composed of an impeller 1, a rotation reduction unit 2, a circular plate 3, a rotation detection unit 6 and an arithmetic unit 7. The rotation detection unit 6 has a first coil 4a, a second coil 4b, a first capacitor 5a and a second capacitor 5b. 
The impeller 1 is placed in a water pipe and rotates at a rotation speed proportional to a flow rate (an amount of water flowing through the water pipe per unit time) of the water. The rotation speed of the impeller 1 is reduced by the rotation reduction unit 2 and transferred to a rotating shaft running through a center of the circular plate 3. The rotation reduction unit 2 is formed including a small gear 2a having a smaller number of teeth and a large gear 2b having a larger number of teeth and meshing with the small gear 2a, and its reduction rate is determined by a ratio between the number of teeth of the small gear 2a and the number of teeth of the large gear 2b. The number of gears included in the rotation reduction unit 2 and the number of teeth of each of the gears may be increased or decreased in accordance with the reduction rate required. As a result, the circular plate 3 rotates at a rotation speed (one rotation per second, for example) that is significantly smaller than the rotation speed of the impeller 1.
The circular plate 3 is composed of a metal portion 3a (made of copper, for example) in a semicircular shape disposed on its principal surface and an insulator portion 3b (made of resin, for example) in a semicircular shape disposed on the rest of the principal surface other than the metal portion 3a, as shown in FIG. 15.
The first coil 4a and the second coil 4b are disposed above the circular plate 3. The first coil 4a and the first capacitor 5a form a first LC resonant circuit, while the second coil 4b and the second capacitor 5b form a second LC resonant circuit.
An oscillation signal is generated when the first LC resonant circuit is activated by applying an activation pulse, for example. Attenuation of the oscillation signal differs depending on whether the first coil 4a is above the metal portion 3a or above the insulator portion 3b. That is, when the first coil 4a is above the metal portion 3a, the first coil 4a loses more energy because an eddy current is caused in the metal portion 3a due to electromagnetic induction by the first coil 4a. As a result, the oscillation signal of the first LC resonant circuit attenuates relatively fast.
When the first coil 4a is above the insulator portion 3b, on the other hand, the attenuation of the oscillation signal of the first LC resonant circuit is determined by internal resistances of the coil, transistor, capacitor and the like, and is relatively slow because no eddy current is caused. Therefore, by periodically sampling a change in the attenuation of the oscillation signal, it is made possible to obtain information on a location of the first coil 4a above the circular plate 3 (that is, whether the coil 4a is above the metal portion 3a or above the insulator portion 3b) at each of the sampling times. The arithmetic unit 7 finds the rotation speed of the circular plate 3 from a change in the information on the location of the first coil 4a over the time.
Then, the arithmetic unit 7 calculates the rotation speed of the impeller 1 from the rotation speed of the circular plate 3 and the reduction rate of the rotation reduction unit 2. The arithmetic unit 7 also calculates the flow rate of the water from known correlation between the rotation speed of the impeller 1 and the flow rate of the water. The arithmetic unit 7 is a microcomputer, for example.
It is understood that the rotation speed of the circular plate 3 can be obtained with the first LC resonant circuit only when it is determined based on the measurement principle as described above. However, it is made possible to detect not only the rotation speed but also a direction of the rotation of the circular plate 3 at the same time by using both the first and second LC resonant circuits. In this case, a minimum sampling rate to detect the location by the first and second LC resonant circuits is represented by Equation (1):MinimumSampling Rate=2×360°/α×RVmax  (1)where α denotes an angle between the first coil 4a and the second coil 4b, that is, an angle formed by a line connecting the first coil 4a and the center O of the circular plate 3 and a line connecting the second coil 4b and the center O of the circular plate 3. RVmax denotes a maximum rotation speed of the circular plate 3.
When α is 90°, for example, the minimum sampling rate is 8×RVmax. Assuming that the maximum rotation speed of the circular plate 3 is one rotation per second, the minimum sampling rate comes to be 8/second.
Next, a concrete structure of the rotation detection unit 6 is explained referring to FIG. 16. Since the first and second LC resonant circuits have the same structure, only a portion including the first LC resonant circuit is explained below.
As shown in the drawing, the rotation detection unit 6 is formed to include an activation transistor 10 made of a P-channel type MOS transistor, a resistor R1 for current limiting, the first coil 4a, the first capacitor 5a, a comparator 11, a latch circuit 12, a power supply line 13 and a capacitor 14 for smoothing. The first LC resonant circuit is formed by connecting the first coil 4a and the first capacitor 5a in parallel.
When the activation pulse GP of an L level (0 V) is applied to the activation transistor 10, the activation transistor 10 is turned on for a period corresponding to a pulse width tw of the activation pulse GP, as shown in FIG. 17. The activation pulse GP is periodically generated in accordance with the sampling rate. When the activation transistor 10 is turned on accordingly, a current is supplied from the power supply line 13 to the first LC resonant circuit through the activation transistor 10 to activate the first LC resonant circuit so that the first oscillation signal is generated at a node N. An oscillation frequency fosc of the first oscillation signal is represented by Equation (2):fosc=½π×√{square root over (1/LC)}  (2)where L denotes an inductance of the first coil 4a, and C denotes a capacitance of the first capacitor 5a. 
The comparator 11 compares the first oscillation signal with a reference voltage Vref. A center voltage of the first oscillation signal is set to 0.5 VDD, while the reference voltage Vref is set to a voltage between 0.5 VDD and VDD. Thus, an output from the comparator 11 becomes an H level when the first oscillation signal is larger than Vref, and becomes the L level when the first oscillation signal is smaller than Vref. The output from the comparator 11 makes a pulse train.
The latch circuit 12 latches the pulse outputted from the comparator 11 in response to a latch pulse RP. The latch circuit 12 is structured so that it latches the pulse outputted from the comparator 11 in response to the latch pulse RP that is generated during a measuring period t2 after a predetermined delay time t1 from the generation of the activation pulse GP.
FIG. 17 shows the first oscillation signal in the case where the first coil 4a is above the insulator portion 3b, thus the attenuation of the first oscillation signal is relatively slow. Therefore, there is a period of time within the measuring period t2 during which the first oscillation signal is higher than the reference voltage Vref. Since the comparator 11 outputs the pulse during the period, the latch circuit 12 latches the pulse and holds data “1” (H level).
When the first coil 4a is above the metal portion 3a, on the other hand, the attenuation of the oscillation signal is relatively fast. As a result, the latch circuit 12 holds data “0” (L level), since the oscillation signal attenuates to a voltage lower than the reference voltage Vref in the measuring time t2 and the comparator 11 does not output the pulse.
Therefore, the rotation detection unit 6 including the first and second LC resonant circuits is capable of identifying four rotation states (a)-(d), as shown in FIGS. 18 and 19.
In the rotation state (a), the first coil 4a is above the metal portion 3a, while the second coil 4b is above the insulator portion 3b. At that time, the first oscillation signal from the first LC resonant circuit attenuates faster than the second oscillation signal from the second LC resonant circuit. Therefore, the data held by the latch circuits 12 in the first and second LC resonant circuits is represented as (0, 1).
In the rotation state (b), both the first coil 4a and the second coil 4b are above the metal portion 3a. At that time, both the first oscillation signal and the second oscillation signal attenuate fast. Therefore, the data held by the latch circuits 12 in the first and second LC resonant circuits is represented as (0, 0).
In the rotation state (c), the first coil 4a is above the insulator portion 3b, while the second coil 4b is above the metal portion 3a. At that time, the second oscillation signal attenuates faster than the first oscillation signal. Therefore, the data held by the latch circuits 12 in the first and second LC resonant circuits is represented as (1, 0).
In the rotation state (d), both the first coil 4a and the second coil 4b are above the insulator portion 3b. At that time, both the first oscillation signal and the second oscillation signal attenuate slowly. Therefore, the data held by the latch circuits 12 in the first and second LC resonant circuits is represented as (1, 1).
Therefore, it is possible to find a rotation period T of the circular plate 3 based on the change in the data held by the two latch circuits 12 over time. The rotation speed of the circular plate 3 is 1/T that is an inverse of the rotation period T. Since single sampling gives the data showing one of the four rotation states (a)-(d), measurement precision of the rotation speed of the circular plate 3 can be increased by increasing the sampling rate.
It is also possible to determine the direction of the rotation of the circular plate 3 base on the order of appearance of the four rotation states (a)-(d). That is, when the rotation states appear in the order (a)→(b)→(c)→(d), it is understood that the circular plate 3 rotates counterclockwise, and when the rotation states appear in the order (d)→(c)→(b)→(a) to the contrary, it is understood that the circular plate 3 rotates clockwise, as shown in FIG. 19.
However, the water meter described above has a problem of high cost because it uses expensive coils. It also has a problem that the LC resonant circuit malfunctions when a permanent magnet is brought close to the coil.
In addition, a spike current flows through the power supply line 13 because the activation transistor 10 is instantaneously turned on by applying the activation pulse GP when the LC resonant circuit is activated. The spike current causes a voltage change in the power supply line 13, which may cause malfunctioning of the circuit. Therefore, the smoothing capacitor 14 of a large capacitance is required to suppress the voltage change.
Furthermore, a reference clock of high frequency is required in order to precisely control the pulse width tw of the activation pulse GP so that the LC resonant circuit is activated. Since the pulse width tw of the activation pulse GP is about 0.8 μsec, the reference clock of about 10 MHz is required. Precisely controlling the delay time t1 described above also requires the reference clock of high frequency.
An objective of this invention is eliminating the need for the coils to reduce the number of external parts so that the cost is reduced. Another objective of this invention is offering a water meter and more generally a fluid flow rate measuring device that requires neither the smoothing capacitor 14 of large capacitance nor the high frequency reference clock by suppressing the spike current.